As is known, isocyanates are produced by the reaction of amines with phosgene. The reaction takes place through formation of the carbamic acid chlorides, which split up into the corresponding isocyanates and hydrogen chloride at elevated temperature. If the boiling point of the isocyanate to be produced lies clearly above the decomposition temperature of the carbamic acid chloride, then the hydrogen chloride formed during decomposition may be readily removed from the reaction vessel, particularly when an inert organic solvent is used. However, if the decomposition temperature of the carbamic acid chloride lies around or above the boiling point of the isocyanate, then the isocyanate is present in the gas evolved and will recombine with the hydrogen chloride to form carbamic acid chloride. Decomposition is accordingly incomplete. The resultant isocyanate is obtained in small yields and is contaminated with carbamic acid chloride. This recombination is particularly troublesome in producing C.sub.1 to C.sub.3 aliphatic monoisocyanates with the greatest difficulties being in the production of methyl isocyanate.
Several processes for overcoming these difficulties have been described. A large number of these processes involve the splitting of carbamic acid chlorides in the presence of hydrogen chloride acceptors.
It is thus known to produce isocyanates from carbamic acid chlorides in organic solvents in the presence of organic bases, such as tertiary amines, carboxylic acid dialkyl amides as described in German Offenlegungsschrift No. 1,593,554 or tetra-alkyl ureas as described in U.S. Pat. No. 3,644,461. Moreover, the use of water is described in German Auslegeschrift No. 2,156,761, and the use of aqueous solutions or suspensions of inorganic bases is described in British Pat. No. 1,208,862, both of these types of materials being described for the absorption of the hydrogen chloride. Also, olefins are described as hydrogen chloride acceptors, in German Offenlegungsschrift No. 2,210,285.
All of these processes have the serious disadvantage that corrosive organic or inorganic salts or alkyl chlorides are formed as by-products which must be further treated in an expensive manner to avoid environmental pollution. In addition, the use of organic bases involves the risk of secondary reactions which lead to the formation of dimeric and trimeric isocyanates. Additionally, a considerable proportion of the carbamic acid chloride is hydrolyzed to the amine-hydrochloride in the presence of water. In general, high yields are only obtained in the case of the relatively inert, tertiary butyl isocyanate.
It is also known to produce low-boiling aliphatic monoisocyanates by thermal splitting of carbamic acid chlorides in organic solvents, by applying special processing techniques.
According to German Auslegeshcirft No. 1,193,034, the thermal splitting of the carbamic acid chloride is carried out in a reactor provided with a reflux condenser and separating column. Hydrogen chloride escapes through the reflux chamber, and isocyanate, carbamic acid chloride and solvent are retained. The isocyanate formed enters the separating column and may be taken off at the head of the column. The majority of the isocyanate is recycled through a reflux divider so that the hydrogen chloride rising in the column is completely absorbed and returns to the reactor in the form of carbamic acid chloride. When this process is carried out continuously, a portion of the solution with reduced carbamic acid chloride content is generally continuously removed from the reactor, enriched with carbamic acid chloride at another point and recycled to the reactor.
German Offenlegungsschriften Nos. 2,411,441; 2,411,442; 2,422,211 and 2,503,270 are typical of prior attempts to thermally split carbamic acid chlorides requiring the use of specific apparatus.
Although it is possible to produce low-boiling aliphatic monoisocyanates by thermal splitting of carbamic acid chlorides by the processes known and described, serious disadvantages are generally observed:
(1) the separation of hydrogen chloride requires reflux condensers with large cooling surfaces which must operate with coolants, consuming a great deal of energy to ensure that isocyanate and carbamic acid chloride are retained quantitatively;
(2) highly efficient fractionation columns are generally required for separating carbamic acid chloride-free isocyanate from the reaction mixture by distillation, since a high reflux ratio must be maintained;
(3) the processes can only be used if relatively dilute carbamic acid chloride solutions having a concentration of 1 to 30% are used; and
(4) when the process is carried out continuously, which is generally necessary for commercial applications, the reaction solution must be circulated several times.
All of these disadvantages necessarily require the reactants (isocyanate, carbamic acid chloride and solvent) to be evaporated, condensed or cooled, and reheated several times in the process, thus giving rise to high energy consumption. A long residence time (and, thus a low space-time yield) and the need for many cycles generally follows from the use of dilute solutions. Because of the long residence time, the yield can even be further reduced by trimerization of the monoisocyanate. The process generally requires high expenditure for measurement and control equipment. A relatively high investment for commercial production necessarily results due to the low space-time yield and the need to use highly efficient fractionation columns.
In addition, it is known to produce isocyanates by thermal splitting of carbamic acid esters (Houben-Weyl, Methoden der org. Chemie; Volume 8, page 126, 1952). In this process, carbamic acid aryl esters are preferably used since they split up into isocyanates under milder conditions than do alkyl esters.
Processes for the production of monoisocyanates, in which both carbamic acid aryl esters and carbamic acid alkyl esters are used, are known and described in the literature.
Thus, the corresponding monoisocyanates may be produced by thermal decomposition from N-alkylcarbamic acid-2-hydroxyethyl esters as described in U.S. Pat. No. 3,076,007 and from N-alkylcarbamic acid-.beta.-naphthylesters as described in German Offenlegungsschrift No. 2,512,514.
In these processes, as in the thermal splitting of carbamic acid chlorides, there is the danger that the cleavage products will recombine to reform the starting materials.
According to the process described in U.S. Pat. No. 3,076,007, recombination is prevented by immediately condensing and chilling the decomposition products. The distillates obtained are then diluted with an inert solvent which is immiscible with water. The ethylene glycol can then be removed by repeated extraction with water. This complicated separation of the decomposition products by extraction is not necessary in the process described in German Offenlegungsschrift No. 2,512,514, since the .beta.-naphthol forming during decomposition does not pass into the gaseous phase under the decomposition conditions due to its high boiling point. However, the thermal stress of the .beta.-naphthol is disadvantageous in this case since it causes the formation of undesirable pyrolysis products. This situation is particularly detrimental when the process is carried out continuously because the reused .beta.-naphthol is enriched with impurities which inevitably interfere in the process for the production of the isocyanates. Another disadvantage of the process is that the separation of the isocyanates is slowed down as the decomposition of the carbamic acid esters progresses so that the increase in concentration of .beta.-naphthol taking place with decomposition shifts the equilibrium between .beta.-naphthol, isocyanate and carbamic acid ester according to the principle of mass action.
An object of the present invention is to provide a process for the continuous production of monoisocyanates in which undesirable recombinations, separation problems and unnecessary thermal stresses on the decomposition products formed by thermal decomposition of N-alkyl carbamic acid aryl esters are avoided.
This object could be achieved by the process according to the invention, which is described below.